1. Field of the Invention
The present invention relates to a mask aligner with a wafer position detecting device, such as a video image pick-up device, for use in a semiconductor circuit manufacturing system. In general, the semiconductor circuit manufacturing process utilizes an automatic positioning device for aligning a wafer with a mask with high accuracy when the wafer is exposed to the pattern of a semiconductor integrated circuit on a photo-mask or reticle (hereinafter both called "mask" (for both).
2. Description of the Prior Art
In the prior art, when a wafer is to be automatically aligned with a mask in the semiconductor exposing device, the outer shape of the wafer, for example, the outer shape of a circular plate partially cut is mechanically utilized to preliminarily align the wafer with the mask as is known by U.S. Pat. No. 3,982,627. This is called "pre-alignment". An optical pre-alignment also is known as described in IBM Technical Disclosure Bulletin, Vol. 14, No. 11, April 1972, pp. 3239-3242. After the pre-alignment, the wafer is moved to an exposure station by means of a moving arm. At the exposure station, the respective alignment marks on the mask and wafer are photoelectrically sensed to generate signals, in accordance with which one of the mask or wafer is moved in an orthogonal direction and rotated.
Methods for photoelectrically detecting the alignment marks include a method in which the alignment marks on the mask and wafer are optically and one-dimensionally scanned by, for example, a laser beam, and the light reflected by the alignment marks is received by a photocell, a method in which the alignment marks are detected by a TV camera to generate video signals which are in turn processed electrically to know the relative position of the alignment marks, and so on.
Upon setting the position in the pre-alignment, which is mechanical, the accuracy is at most on the order of .+-.100 .mu.m. This relatively low accuracy should be compensated with a photoelectric detector having its wide field of view which is above several hundreds of microns. As the field of view is wider on detection, however, time required to detect signals is extended. This is particularly so, since a plurality of scanning operations must be done. Furthermore, the wider field of view involves difficulties in that the accuracy of detection is limited when an optical system of wide field of view cannot easily be improved in performance, and so on. If the positional deviation between the wafer and the mask is increased at the initial condition, time required to automatically align the wafer with the mask is remarkably extended. On the other hand, a typical exposing apparatus, particularly of such a type that a circuit pattern on the mask is projected on the wafer through a projection optical system, utilizes a TTL system for detecting the alignment marks through the projection optical system, or an Off-Axis method in which the wafer is precisely positioned at a position out of the projection field (a region on which an image is projected) of the projection optical system and then moved to the projection field with high accuracy to align the wafer with the mask which has properly been set in the projection field. However, the former has a problem associated with the wider field of view afore-mentioned while the latter requires a very expensive gauge interferometer and precise transport mechanism for moving the wafer with high accuracy.